The invention relates to a power steering system for a vehicle having one or more steerable wheels, and which comprises a manually operable steering wheel, a first angle sensing device connected to the steering wheel, a servo motor connected to the steerable wheel or wheels and including a second angle sensing device, and a control unit for receiving signals from the first angle sensing device and for energizing the servo motor.
In prior art systems of this kind, there are used analog types of angle sensing means like potentiometers or DC-tachometers connected to the steering wheel. In those systems, it is fairly simple to obtain a reliable safety control of the angle sensing device operation. Such a safety control is hightly important, since it is crucial for the safety of the vehicle operation.
For different reasons it has become desirable to use digital angle sensing devices instead of the former analog devices both as a steering wheel signal producer and as an angle controlling device at the steerable wheel of the vehicle, and according to one previously suggested solution to the safety problem two separate digital angle encoders have been used. A faulty operation of one of these encoders indicates a malfunction to be attendted to, but because two encoders are provided, the steering system would remain operative to enable a safe handling of the vehicle to stand still.
It is desirable, however, to simplify the power steering system and to bring down its costs, and a solution to that would be to use just one digital angle encoder. There is a problem, though, to accomplish a reliable detection of a faulty operation of such system and to initiate an immediate stop to the vehicle operation.
In a power steering analog system described in EP 0 885 756, there are used analog type angle sensors in connection with the steerable wheels of the vehicle, and there is also described a method for faulty detection of these sensors.
However, in this known system there is used a digital angle encoder as a signal generating means in connection with the steering wheel. However, there is not described or mentioned anything about how to accomplish a reliable function monitoring of this digital angle encoder.
According to the invention, a power steering system for a vehicle having one or more steerable wheels comprises a steering wheel which is mechanically connected to a single digital angle encoder, a servo motor with an angle sensing device connected to the steerable wheels, and a malfunction detecting device connected to the digital angle encoder and arranged to apply a constant analog voltage across the digital encoder to, thereby, alter the digital pulse amplitude from the encoder to a certain check amplitude, wherein a faulty angle encoder operation is detectable as a deviation of the pulse amplitude from that check amplitude.
The above described problems are solved by the invention which provides an improved power steering system including a simplified but safely controlled digital angle sensing device.
A preferred embodiment of the invention is described below with reference to the accompanying drawings.